Hmm... Let's try it this way. You know what Windows is, right? How you can run programs, get online, etc. through Windows? Windows is like a framework that your programs run in. (A slightly innaccurate analogy, but it'll work for this.)
Well Linux does the same thing Windows does. Except where you have to pay for Windows, Linux is free. And where you have to worry about viruses and spyware in Windows, Linux is a lot more secure. (Well, except for Lindows, which was a Linux that tried to be more like Windows...) And where there are only a couple or three versions of Windows, there is a whole universe of Linux variants, all made for different kinds of people - some for people who want a minimal desktop and more power for programs, some for people who want something that works without having to know a lot about computers, more than one that is made primarily for watching tv online, lots of Linux that are made for music makers... mmmm, a whole computer system just for us music makers.
Also, since Linux is free, the makers are free to do a lot of things that Microsoft might do with Windows, but if they did it Microsoft would lose money. For example, you know how you buy a copy of Windows that comes on a DVD and you use the DVD to install Windows on your computer? Many kinds of Linux (mostly the more user-friendly ones) can run from the DVD without install. As in, you don't have to wipe out what is on your computer in order to try Linux, you just run the whole system from the DVD. In fact you could download lots of different kinds of Linux, burn them onto DVDs and try them all, without messing up the Windows that is installed on your computer.